Rare earth doped optical waveguides are attracting more and more interest for use as lasers and amplifiers. The extension of rare earth doping to integrated optics has recently received considerable attention due to the potential for mutiple-device fabrication on a single substrate.
In one approach, glass waveguides are made by ion exchange techniques made directly in the substrate. For example, an ion exchange with a rare earth doped substrate results in a laser or amplifier. However, this approach demands careful selection and control of the substrate glass composition and processing techniques to obtain desirable low loss waveguides. In addition, since the rare earth doping is not limited to selected sites or channels but exists throughout the substrate, low loss passive elements cannot be fabricated along with the active laser or amplifier elements in three-level rare earth systems due to the attenuation in the unpumped regions. Erbium is a three-level system and is a preferred dopant because it exhibits optical gain in the 1.5 micron region. For reasons stated previously, ion exchange is not a good method for fabricating devices based on erbium doped waveguides.
In another approach, flame hydrolysis deposition (FHD) and a solution doping technique used in creating rare earth doped optical fibers, is applied to make rare earth doped planar waveguides. FHD is used to deposit a layer of unconsolidated silica (SiO.sub.2) soot on a planar substrate. The wafer is then emersed in a solution containing rare earth ions, removed and dried. The soot is then consolidated to include the rare earth oxide and create on the substrate a rare earth doped glass which can be formed into active optical components such as lasers and amplifiers. One problem with this approach is that the distribution of the rare earth in the consolidated soot is not very uniform. This is so because the uniformity depends on the diffusion of the rare earth during consolidation when in fact rare earth elements are not very mobile. Non-uniformity is problematic because it causes clustering and it lends itself to concentration quenching from high local concentration of rare earth.